Degradable plastic is considered to be one of the most promising green packaging materials. It is a new type of plastic that can quickly degrade and disappear in the natural environment after being discarded without causing environmental pollution.
According to the definition established by ASTM in 1989, degradable material refers to a plastic whose chemical structure changes in a specific environment that causes performance loss within a specific time. The development of degradable plastic packaging materials and the gradual elimination of non degradable plastic packaging materials are the general trend of the development of science and technology in the world and one of the hot spots of material research and development. Degradable plastics are easy to process and form, and the price is decreasing day by day, which has led to a sharp increase in the use of degradable plastics for packaging. According to the U.S. market and technology forecast, more than 40 companies in the United States have produced degradable plastics, with an output of 2. 5% in 1987 30000 tons, increased to 380000 tons in 1992, an increase of 17 times in five years. At present, degradable plastics have been widely used in food packaging, turnover boxes, grocery bags, tool packaging and the outer packaging of some mechanical and electrical products in the United States.
At present, the only fully biodegradable polymers that have been confirmed are biosynthetic aliphatic polyesters, such as PHB and PHBV synthesized by fermentation, polymers containing ether bonds and polyhydroxy groups such as starch and cellulose, as well as synthetic polyamides, polyurethane, polymers containing ether bonds and PVA. Among these polymer materials, polyurethane, polyamide, polyester and PVA are all packaging materials with good performance, which are widely used as packaging films, packaging containers or binding materials. However, these completely degradable packaging materials have limited varieties, and are far from meeting the requirements of many packaging technologies and packaging protection properties. PHB, PHBV and polyether are not very useful because of their low melting point and strength. However, packaging materials that can be completely degraded by microorganisms have attracted much attention from all countries because of their complete harmlessness to the environment, and the development of this aspect is still in progress.
The incomplete polymer degradable plastic (starch +pe type) blended with starch, also known as bio splitting plastic, is a material with rapid research and development, many industrialization achievements, and is expected to continue to reduce costs. In order to improve the blending compatibility between starch and polymers, starch must be modified. The surface of modified starch particles is covered by alkyl, which weakens the role of hydrogen bond, and thus increases the compatibility with polyethylene, polyvinyl alcohol and other polymers.
China began to develop degradable starch plastics in the early 1980s. The starch plastics developed by the Yinghua Institute of Jiangxi Academy of Sciences first achieved success. In 1987, it passed the appraisal and established the first small starch plastic production line in China. The product performance reached the international standard of the same type of plastics. Beijing Huaxin starch degradable resin products Co., Ltd. used starch as the main raw material instead of some polyethylene as raw material, and put the degradable food packaging bags on the market in 1991.
The problem of starch degradable plastics is that during degradation, only the starch part is biodegradable in a short time, while the plastic part, such as polymers such as PK, PS, PVC, etc., cannot be biodegradable. The degradation result is that although the blend is degraded and its performance is degraded, it is still left in the soil or environment. It will take at least 20 years to achieve complete degradation and enter the ecological environment. Therefore, this is a kind of incomplete biodegradable materials, It can only play a moderating role in the treatment of plastic waste, rather than fundamental treatment.
Photodegradation packaging materials obtained by adding photosensitizers have been reported. PE, PP and other commonly used plastics can be obtained by adding a suitable photosensitizer. TPR green packaging material developed by filltec company in Sweden is made by compounding calcium carbonate with polypropylene added with photolysis agent through special process. Its composition is very similar to egg shell and is almost harmless to the environment. It can be thermoformed, blow molded, injection molded and extruded. TPR is smooth and smooth. Films of different thickness degrade into powder after 4-18 months under light. It is now used in the packaging of butter, ice cream, etc.
At present, photodegradation packaging materials still have the following problems: 1) the contradiction between photodegradation speed and the performance of photodegradable polymers. If the photodegradation speed is too fast, although it is conducive to the treatment of waste plastics, beautify the environment and reduce pollution, it is obviously detrimental to the service performance and service life; 2) The impact of photodegradation products on the ecological environment, if the photodegradation products can continue to undergo biodegradation, and finally become carbon energy or harmless substances absorbed by microorganisms, of course, it is the best. But in fact, the copolymers of ethylene, propylene and vinyl ketone have very little biodegradation ability in soil or underground waterways after photodegradation, and whether the photodegradation products are harmful to the ecological environment is also worth studying.
At present, biodegradable and photodegradable plastics entering the market are mainly obtained by blending or grafting general-purpose polymers such as PE and PP with degradable polymers such as starch or cellulose, and adding photosensitizers that can induce photodegradation. The research and application in this field are very active. Polyketones can be chemically modified with oxidants such as hydrogen peroxide and peroxy acid, and oxidized into polymers containing polyester and polyketone structures at the same time, becoming packaging materials with both biodegradability and photodegradation. The biodegradable / photodegradable plastic developed by the Department of chemistry of Lanzhou university can be directly used in the production of fast-food lunch boxes and garbage bags. It uses non starch photosensitizers and biodegradable agents. Its strength and transparency are better than starch plastic. It has excellent photodegradation performance and can be embrittled within 50~100 days. Its degradation products can be further degraded by microorganisms such as mold, and finally become the carbon source of microorganisms and return to nature. The main materials of Shuang Jie plastic produced by Guangdong Shenzhen Huipeng resin factory are PF and starch, of which the content of modified starch is up to 70%. Blow molding is used for packaging, and this film can be degraded under biological and light effects. In recent years, many countries have strengthened the deep processing and development of agricultural and sideline products in order to improve the comprehensive benefits of agriculture. Using agricultural and sideline products to produce environmental friendly food plastic packaging materials has become a research hotspot in the field of scientific research.
At present, the application of "biodegradable resin" to replace the existing packaging plastics is in full swing. Corn is a delicious and nutritious food. It is also widely used in the manufacture of sweeteners and animal feed. With the progress of technology, it can also be used to produce a variety of plastic products. For example, the "corn starch resin" successfully studied in Japan and Taiwan is a new type of green environmental protection material. This resin is made from corn by processing and plasticizing. First, the sugar in corn is extracted, After fermentation and distillation, the basic materials for manufacturing plastics and fibers are extracted, and then processed into a material called polylactide (PLA), which is a fine granular material with a diameter of only 4.57mm. It can be made into a variety of disposable products, such as water cups, plastic bags, commodity packaging, etc. According to experiments, this packaging material can be disposed of by burning, biochemical decomposition and insect eating, so as to avoid "white pollution".
Foreign companies have been optimistic about this new environmental protection material. For example, Coca Cola used 500000 disposable cups at the Salt Lake City Winter Olympics, all made of corn plastic. This kind of cup disappeared in the open air in only 40 days. Sony, a Japanese electrical appliance manufacturer, has been packaging MD trays with plastic paper made of corn for two years. The new packaging is as beautiful as the previous packaging, but it will not produce the adverse effect of "durability".
Scientists from the U.S. Department of Agriculture recently announced that a new environmental friendly food packaging material is expected to come out. This new material is completely made of crushed strawberries, which meets the requirements of environmental protection and may replace the traditional polyethylene plastic as a new food packaging material. Tara McHugh, a food expert from the U.S. Department of agriculture, said that this newly developed food packaging material is no different from traditional products in terms of performance. The food packaging film made of it can also prevent the penetration of oxygen, so as to achieve the effect of food preservation. Since the ingredients of this new material are mainly from inferior vegetables and fruits that are not suitable for eating, it is a material that can be naturally decomposed and meets the requirements of environmental protection. In the test, the packaging film made of this "magical" material can not only keep fresh, but also improve the taste of bananas, apples and other fruits. Tara mchuey said that in the near future, not only strawberries can be used as packaging materials, but also vegetables such as carrots and cauliflower can be used as packaging materials.
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